To assess the relative contribution of the chordal structures and mitral valve apparatus to left ventricular function (LVF), in the presence of chronic mitral regurgitation (CMR), 18/26 animal models of CMR were developed by creating a 4mm perforation of the anterior mitral valve leaflet during cardiopulmonary bypass. Complete hemodynamic, ultrasonographic, and radionuclide functional analysis will be performed on a control group of animals and in all survivors of the perforation operation to evaluate the effects of CMR on LVF. Data acquired to date show that a 70-80% survivorship was attained in 20-25kg animals, and over 90% in 25-30kg animals. Acute changes in LVF included a drop in dP/dt, while cardiac output remain unchanged. Right heart, left atrial and wedge pressure increased. The second phase of the study consists of performing mitral valve replacement (MVR) in 18 control animals, half of which will be randomized to chordal preservation and half to standard native- valve-removal. The study arm consists of randomization of the CMR animals to these same two groups. Studies of LV function and geometry along with nuclear angiographic data will be obtained and analyzed, to assess whether chordal preservation during MVR has any effect on subsequent LVF in normal volume overloaded ventricles.